Various types of discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a Blu-ray disc can be set in an optical disc apparatus for recording or reproduction. In recent years, dual discs have also been commercially available which have a non-DVD surface, that is, a music-only surface, and a DVD surface laminated on the disc opposite the non-DVD surface so that reflection layers in the non-DVD and DVD surfaces function in opposite directions; digital audio information is recorded on the non-DVD surface, and video information or the like is recorded on the DVD surface on the basis of DVD standards.
Physical standards for CDs limit a maximum tolerance value for a substrate thickness to 1.5 mm. Accordingly, For dual discs, to minimize the thickness of the disc, the DVD surface has a substrate thickness of 0.6 mm, which is the same as that of normal CDs, but the non-DVD surface has a substrate thickness of about 0.9 mm, which is smaller than that of the normal CDs, 1.2 mm. However, since the only difference between the dual disc and the normal CD is the substrate thickness, the non-DVD surface of the dual disc is determined to be a CD in optical disc determination.
To allow a single optical pickup to optically access these discs with the different substrate thicknesses for recording or reproduction, it is necessary to automatically determine what type of disc has been set in the apparatus. The type of the disc is conventionally determined on the basis of a focus balance value (symmetry) obtained by normalizing the deviation of the S-shaped signal symmetry of a focus error signal FE resulting from spherical aberration, on the basis of the amplitude of the focus error signal FE. According to WO 2003/063149 and Japanese Patent Laid-Open No. 2002-245639, the focus balance value is obtained by:(FEmax+FEmin)/(FEmax−FEmin)  Formula 1,and then compared with a reference value to determine the type of the disc. However, FEmax and FEmin in the above formula are values relative to the reference level of the focus error signal FE. Formula 1 can thus be rewritten as:(FEmax−FEmin)/(FEmax+FEmin)  Formula 2
The present invention handles FEmax and FEmin in absolute values. Thus, Formula 2 is used for comparison with the present invention. However, in absolute values, FEmin always has a negative value, so that the results of the formulae are almost the same.
Specifically, an optical disc apparatus of this type is configured as shown in FIG. 8.
An optical pickup 1 can apply recording signals to and obtain reproduction signals from, different types of discs 2, for example, a CD and a DVD. The optical pickup 1 has a semiconductor laser 3, a condensing lens 4, an objective lens 5, a polarizing hologram 6, a tracking detecting light receiving unit 7, and a focus detecting light receiving unit 8. The semiconductor laser 3 is controlled by a laser control unit 9 to emit laser light of appropriate power. To avoid focus errors and tracking errors, the objective lens 5 can be moved in a focus direction and a disc diameter direction, respectively, in response to driving signals from an actuator driving unit 10. The condensing lens 4 converts laser light emitted by the semiconductor laser 3 into parallel light. The light having passed through the condensing lens 4 forms an optical spot on the disc 2 through the objective lens 5.
Reflected light from the disc 2 passes through the objective lens 5 and the condensing lens 4 and is diffracted into reflected light for focus detection and reflected light for tracking detection by the polarizing hologram 6. The reflected light for focus detection is detected by the focus detecting light receiving unit 8.
An FE signal measuring unit 11 measures a maximum value FEmax 12 and a minimum value FEmin 13 of the focus error signal output by the focus detecting light receiving unit 8, in the form of voltage or current values, or the like.
A disc determining unit 14 can determine the type of the disc on the basis of a difference in the waveform of an S-shaped signal using, as a reference, a focus error signal obtained at a position where the laser light can be focused. Here, the type of the disc is determined in accordance with Formula 1 on the basis of the maximum value FEmax and minimum value FEmin of the amplitude of the focus error signal measured by the FE signal measuring unit 11.
A control unit 15 controls the actuator driving unit 10, the disc determining unit 14, and the laser control unit 9.
FIG. 9 is a flowchart for optical disc determination made by the optical disc apparatus shown in FIG. 8.
In step S1, the disc 2 is irradiated with laser light. In step S2, the objective lens 5 in the optical pickup 1 is driven in the focus direction. While the objective lens 5 is driven, the FE signal is measured in step S3, the maximum value of the FE signal is stored in FEmax in step S4, and the minimum value of the FE signal is stored in FEmin in step S5.
In step S6, the difference between the absolute value of FEmax and the absolute value of FEmin is compared with a predefined determination value.
In step S6, if the difference is greater than the predefined determination value, the apparatus determines in step S7 that the set disc 8 is a “CD”. If the difference is smaller than the predefined determination value, the apparatus determines in step S8 that the set disc 8 is a “DVD”.